Potential adhesion mechanisms for localisation of haemopoietic progenitors to bone marrow stroma

Hematopoietic Niche - Exploring Biomimetic Cues to Improve the Functionality of Hematopoietic Stem/Progenitor Cells

The adult bone marrow (BM) niche is a complex entity where a homeostatic hematopoietic system is maintained through a dynamic crosstalk between different cellular and non-cellular players. Signaling mechanisms triggered by cell-cell, cell-extracellular matrix (ECM), cell-cytokine interactions, and local microenvironment parameters are involved in controlling quiescence, self-renewal, differentiation, and migration of hematopoietic stem/progenitor cells (HSPC). A promising strategy to more efficiently expand HSPC numbers and tune their properties ex vivo is to mimic the hematopoietic niche through integration of adjuvant stromal cells, soluble cues, and/or biomaterial-based approaches in HSPC culture systems. Particularly, mesenchymal stem/stromal cells (MSC), through their paracrine activity or direct contact with HSPC, are thought to be a relevant niche player, positioning HSPC-MSC co-culture as a valuable platform to support the ex vivo expansion of hematopoietic progenitors. To improve the clinical outcome of hematopoietic cell transplantation (HCT), namely when the available HSPC are present in a limited number such is the case of HSPC collected from umbilical cord blood (UCB), ex vivo expansion of HSPC is required without eliminating the long-term repopulating capacity of more primitive HSC. Here, we will focus on depicting the characteristics of co-culture systems, as well as other bioengineering approaches to improve the functionality of HSPC ex vivo.

Isolation of Mouse Bone Marrow Mesenchymal Stem Cells

Mesenchymal stem cells (MSCs) were initially characterized as connective tissue progenitors resident in bone marrow, but have now been isolated from a variety of tissues and organs and shown to also exhibit potent tissue regenerative properties mediated largely via paracrine actions. These findings have spurred the development of MSC-based therapies for treating a diverse array of nonskeletal diseases. Although genetic and experimental rodent models of disease represent important tools for developing efficacious MSC-based therapies, development of reliable methods to isolate MSCs from mouse bone marrow has been hampered by the unique biological properties of these cells. Indeed, few isolation schemes afford high yields and purity while maintaining the genomic integrity of cells. We recently demonstrated that mouse MSCs are highly sensitive to oxidative stress, and long-term expansion of these cells in atmospheric oxygen selects for immortalized clones that lack a functional p53 protein. Herein, we describe a protocol for the isolation of primary MSCs from mouse bone marrow that couples immunodepletion with culture in a low-oxygen environment and affords high purity and yield while preserving p53 function.

The ins and outs of hematopoietic stem cells: studies to improve transplantation outcomes

Deciphering the mechanisms of hematopoietic stem/progenitor cell (HSPC) mobilization and homing is important for the development of strategies to enhance the efficacy of HSPC transplantation and achieve the full potential of HSPC-based cellular therapy. Investigation of these mechanisms has revealed interdependence among the various molecules, pathways and cellular components involved, and underscored the complex nature of these two processes. This review summarizes recent progress in identifying the specific factors implicated in HSPC mobilization and homing, with emphasis on our own work. Particularly, we will discuss our studies on stromal cell-derived factor-1 and its interaction with its receptor CXCR4, proteases (matrix metalloproteinases and carboxypeptidase M), complement proteins (C1q, C3a, C5a, membrane attack complex), sphingosine-1-phosphate, and pharmacologic agents such as the histone deacetylase inhibitor valproic acid and hyaluronic acid.

Evidence for a regulatory role of alpha 4-integrins in the maturation of eosinophils generated from the bone marrow in the presence of dexamethasone

BACKGROUND

Although eosinophils co-express multiple integrin receptors, the contributions of integrins to eosinophil development have not been explored. We previously described extensive aggregation and cytological immaturity in eosinophils developing in bone-marrow (BM) cultures exposed to dexamethasone. Here we examined the relationship of alpha 4 integrins with these effects of dexamethasone.

OBJECTIVES

We evaluated: (a) the effects of exposure to dexamethasone in BM culture on eosinophil expression of alpha 4 integrin receptors and ligands; (b) the contribution of alpha 4 integrins to eosinophil aggregation and maturation.

METHODS

Cultures were established with IL-5 (alone or with dexamethasone) for up to 7 days, and eosinophil production, alpha 4 integrin receptor/ligand expression, aggregation and morphology were evaluated before and after targeting alpha 4 integrin-dependent adhesions. Because prostaglandin E2 (PGE2) modifies the effects of dexamethasone on eosinophilopoiesis, PGE2 effects on alpha 4 integrin expression and function were also evaluated.

RESULTS

Dexamethasone increased the yield of eosinophils up to day 7. The frequency of eosinophils expressing alpha 4, beta1 and beta 7 integrin receptors at day 7 was also increased by dexamethasone. Eosinophils also expressed the alpha 4 beta 1 ligand, VCAM-1. Dexamethasone increased the expression of alpha 4 integrin and VCAM-1 in aggregates containing eosinophils as early as day 3. PGE2, added up to day 3, modified the effects of dexamethasone to suppress the expression of alpha 4 integrin, decrease aggregation and promote cytological maturation of eosinophils recovered at day 7. Dissociation of immature eosinophils from clusters present at day 3 by reagents targeting alpha 4 or beta1 integrins or VCAM-1 also induced cytological maturation. The concordant effects of targeting alpha 4 integrins with drugs and antibodies support a relationship between alpha 4-mediated aggregation and maturational arrest.

CONCLUSIONS

These observations support a novel role for alpha 4 integrin receptors and ligands in eosinophilopoiesis. In addition, increased alpha 4 expression following glucocorticoid exposure may contribute to the retention and accumulation of eosinophils in haemopoietic tissue.

Protein phosphatase 2A plays an important role in stromal cell-derived factor-1/CXC chemokine ligand 12-mediated migration and adhesion of CD34+ cells

Migration of hemopoietic stem and progenitor cells (HSPC) is required for homing to bone marrow following transplantation. Therefore, it is critical to understand signals underlying directional movement of HSPC. Stromal cell-derived factor-1 (SDF-1)/CXCL12 is a potent chemoattractant for HSPC. In this study, we demonstrate that the serine-threonine protein phosphatase (PP)2A plays an important role in regulation of optimal level and duration of Akt/protein kinase B activation (a molecule important for efficient chemotaxis), in response to SDF-1. Inhibition of PP2A, using various pharmacological inhibitors of PP2A including okadaic acid (OA) as well as using genetic approaches including dominant-negative PP2A-catalytic subunit (PP2A-C) or PP2A-C small interfering RNA, in primary CD34(+) cord blood (CB) cells led to reduced chemotaxis. This was associated with impairment in polarization and slower speed of movement in response to SDF-1. Concomitantly, SDF-1-induced Akt phosphorylation was robust and prolonged. Following SDF-1 stimulation, Akt and PP2A-C translocate to plasma membrane with enhanced association of PP2A-C with Akt observed at the plasma membrane. Inhibition of PI3K by low-dose LY294002 partially recovered chemotactic activity of cells pretreated with OA. In addition to chemotaxis, adhesion of CD34(+) cells to fibronectin was impaired by OA pretreatment. Our study demonstrates PP2A plays an important role in chemotaxis and adhesion of CD34(+) CB cells in response to SDF-1. CD34(+) CB cells pretreated with OA showed impaired ability to repopulate NOD-SCID mice in vivo, suggesting physiological relevance of these observations.

Annexin II expressed by osteoblasts and endothelial cells regulates stem cell adhesion, homing, and engraftment following transplantation

Differentiation of hematopoietic stem cells (HSCs) after birth is largely restricted to the bone marrow cavity, where HSCs are associated closely with osteoblasts (OBs). How OBs localize HSCs to the endosteal niche remains unclear. To explore adhesive interactions between HSCs and OBs, a cell blot analysis was used that revealed 2 major bands that corresponded to monomers and multimers of annexin II (Anxa2). Immunohistochemistry revealed that OBs and marrow endothelial cells express Anxa2 at high levels. Function-blocking studies confirmed that Anxa2 mediates HSC adhesion mainly via the N-terminal portion of the Anxa2 peptide. Adhesion of HSCs to OBs derived from Anxa2-deficient animals (Anxa2(-/-)) was significantly impaired compared with OBs obtained from wild-type animals (Anxa2(+/+)). Moreover, fewer HSCs were found in the marrow of Anxa2(-/-) versus Anxa2(+/+) animals. Short-term lodging, engraftment, and survival of irradiated mice with whole marrow cells were substantially inhibited by N-terminal peptide fragments of Anxa2 or anti-Anxa2 antibodies. Similar findings were noted in long-term competitive repopulation studies. Collectively, these findings reveal that Anxa2 regulates HSC homing and binding to the bone marrow microenvironment and suggest that Anxa2 is crucial for determining the bone marrow niche of HSCs.

beta2 Integrins are characteristically absent in acute promyelocytic leukemia and rapidly upregulated in vivo upon differentiation with all-trans retinoic acid

Although little is known about migration of hematopoietic stem cells and their neoplastic counterparts into tissues and peripheral blood, adhesion proteins likely play an important role. We studied 339 patients with acute myelogenous leukemia (AML) to discern the relationship between adhesion protein expression, circulating blasts, and white blood cell (WBC) count. Expression levels of CD11b and CD11c strongly correlated with increased WBC count, independent of FAB subtype (p<0.0001). However, 93% (25/27) of cases of AML-M3 completely lacked beta2 integrin expression, compared to 11% (35/312) of the non-M3 cases (p<0.0001). Seven of the 27 patients with AML-M3 were followed during standard induction therapy with ATRA. Within 3 days, weak CD11c became detectable, followed by CD11b and CD11a. Our data suggest an important link between beta2 integrin expression and the level of circulating leukemic cells in AML. We demonstrate the clinical usefulness of a panel of beta2 integrins (CD11a, CD11b and CD11c) in accurate prediction of AML-M3, and recommend inclusion of this immunophenotypic analysis to identify patients who require ATRA therapy. Finally, we illustrate the rapidity at which AML-M3 blasts up-regulate beta2 integrins, and suggest a possible association between this finding and the tissue infiltration that characterizes the "ATRA syndrome".

Integrin inhibition through Lyn-dependent cross talk from CXCR4 chemokine receptors in normal human CD34+ marrow cells

We studied the effects of Lyn ablation on CXCR4 receptor-mediated migration and adhesion of hematopoietic precursors. Down-regulation of Lyn expression with siRNA greatly reduced CXCR4-dependent hematopoietic cell movement, and increased cell adherence to stroma. This increase was associated with the up-regulated expression of activation-dependent epitopes of the beta(2) integrin LFA-1 and was prevented by antibodies that selectively block cell adhesion mediated by ICAM-1. Attachment to surfaces coated with ICAM-1 was also enhanced in Lyn-depleted hematopoietic cells, as compared with Lyn-expressing cells. Functional rescue experiments with Lyn siRNA targeting the 3' UTR indicated that the observed effects can be attributed directly to specific inhibition of Lyn. Our results show that in chemokine-stimulated hematopoietic cells Lyn kinase is a positive regulator of cell movement while negatively regulating adhesion to stromal cells by inhibiting the ICAM-1-binding activity of beta(2) integrins. These results provide a molecular mechanism for cross talk between the chemokine receptor CXCR4 and beta(2) integrins. This cross talk may allow chemokine receptors to modulate the arrest of rolling hematopoietic precursors on the surface of bone marrow stromal cells.

Myeloid growth factors in oncology

Within the last decade haemopoietic growth factors have become established in the pharmacopoeia of oncology. In the form of granulocyte colony-stimulating factor (G-CSF), and to a lesser extent granulocyte-macrophage colony-stimulating factor (GM-CSF), these proteins are routinely used to accelerate restoration of neutrophil count after chemotherapy or bone marrow transplant. Their main advance has been the development of mobilisation protocols. Peripheral blood progenitor cells are induced to egress from the bone marrow and re-transfusion after myelosuppressive chemotherapy allows for a simple and more rapid form of autologous transplantation than bone marrow transplantation. This review will give a brief overview of the biology of haemopoiesis in relation to growth factors and the potential lines of further research. Although the established clinical uses of G-CSF will be discussed the main focus will be on the developmental applications, such as ex vivo haemopoiesis, dose intensification schedules and the application of growth factors in the therapy of haematological malignancies. The relevance of novel or more recently introduced recombinant haemopoietic growth factors will also be discussed in relation to these indications.

Blood and bone: two tissues whose fates are intertwined to create the hematopoietic stem-cell niche

The mechanisms of bone and blood formation have traditionally been viewed as distinct, unrelated processes, but compelling evidence suggests that they are intertwined. Based on observations that hematopoietic precursors reside close to endosteal surfaces, it was hypothesized that osteoblasts play a central role in hematopoiesis, and it has been shown that osteoblasts produce many factors essential for the survival, renewal, and maturation of hematopoietic stem cells (HSCs). Preceding these observations are studies demonstrating that the disruption or perturbation of normal osteoblastic function has a profound and central role in defining the operational structure of the HSC niche. These observations provide a glimpse of the dimensions and ramifications of HSC-osteoblast interactions. Although more research is required to secure a broader grasp of the molecular mechanisms that govern blood and bone biology, the central role for osteoblasts in hematopoietic stem cell regulation is reviewed herein from the perspectives of (1) historical context; (2) the role of the osteoblast in supporting stem cell survival, proliferation, and maintenance; (3) the participation, if any, of osteoblasts in the creation of a stem cell niche; (4) the molecules that mediate HSC-osteoblast interactions; (5) the role of osteoblasts in stem cell transplantation; and (6) possible future directions for investigation.

Biology of human hematopoietic stem and progenitor cells present in circulation

Circulating hematopoietic stem and progenitor cells play important roles in the physiology and homeostasis of the hematopoietic system. The frequency of these cells varies throughout development, being more abundant during gestation. In the adult, the numbers of such cells are extremely low; however, they can be increased by intravenous administration of chemotherapy and/or recombinant cytokines to individuals. This mechanism--known as mobilization--involves the disruption of the interactions between primitive hematopoietic cells and microenvironment elements (stromal cells and extracellular matrix molecules), which are mediated by a group of molecules known as cell adhesion molecules. During the last two decades, circulating cells of newborns (those present in umbilical cord blood) and adults (mobilized peripheral blood) have gained relevance not only because of their biology, but also because of their clinical application. Indeed, at present the number of mobilized peripheral blood-derived hematopoietic cell transplants performed worldwide is clearly superior to the number of bone marrow transplants being done annually. On the other hand, the number of cord blood transplants has significantly increased during the last few years, and cord blood banking has expanded in a significant manner over the last decade. Circulating stem and progenitor cells are being manipulated ex vivo, both in cellular and molecular terms, and the clinical use of these manipulated cells is just beginning. Undoubtedly, hematopoietic cells present in circulation will play a key role in the development of both gene and cellular therapies for a variety of diseases.

The bone marrow is akin to skin: HCELL and the biology of hematopoietic stem cell homing

The recent findings that adult stem cells are capable of generating new blood vessels and parenchymal cells within tissues they have colonized has raised immense optimism that these cells may provide functional recovery of damaged organs. The use of adult stem cells for regenerative therapy poses the challenging task of getting these cells into the requisite sites with minimum morbidity and maximum efficiency. Ideally, tissue-specific colonization could be achieved by introducing the stem cells intravascularly and exploiting the native physiologic processes governing cell trafficking. Critical to the success of this approach is the use of stem cells bearing appropriate membrane molecules that mediate homing from vascular to tissue compartments. Hematopoietic stem cells (HSC) express a novel glycoform of CD44 known as hematopoietic cell E-/L-selectin ligand (HCELL). This molecule is the most potent E-selectin ligand natively expressed on any human cell. This article reviews our current understanding of the molecular basis of HSC homing and will describe the fundamental "roll" of HCELL in opening the avenues for efficient HSC trafficking to the bone marrow, the skin and other extramedullary sites.

Supplementation of conventional freezing medium with a combination of catalase and trehalose results in better protection of surface molecules and functionality of hematopoietic cells

Our previous studies had shown that a combination of the bio-antioxidant catalase and the membrane stabilizer trehalose in the conventional freezing mixture affords better cryoprotection to hematopoietic cells as judged by clonogenic assays. In the present investigation, we extended these studies using several parameters like responsiveness to growth factors, expression of growth factor receptors, adhesion assays, adhesion molecule expression, and long-term culture-forming ability. Cells were frozen with (test cells) or without additives (control cells) in the conventional medium containing 10% dimethylsulfoxide (DMSO). Experiments were done on mononuclear cells (MNC) from cord blood/fetal liver hematopoietic cells (CB/FL) and CD34(+) cells isolated from frozen MNC. Our results showed that the responsiveness of test cells to the two early-acting cytokines, viz. interleukin-3 (IL-3) and stem cell factor (SCF) in CFU assays was better than control cells as seen by higher colony formation at limiting concentrations of these cytokines. We, therefore, analyzed the expression of these two growth factor receptors by flow cytometry. We found that in cryopreserved test MNC, as well as CD34(+) cells isolated from them, the expression of both cytokine receptors was two- to three-fold higher than control MNC and CD34(+) cells isolated from them. Adhesion assays carried out with CB/FL-derived CD34(+) cells and KG1a cells showed significantly higher adherence of test cells to M210B4 than respective control cells. Cryopreserved test MNC as well as CD34(+) cells isolated from them showed increased expression of adhesion molecules like CD43, CD44, CD49d, and CD49e. On isolated CD34(+) cells and KG1a cells, there was a two- to three-fold increase in a double-positive population expressing CD34/L-selectin in test cells as compared to control cells. Long-term cultures (LTC) were set up with frozen MNC as well as with CD34(+) cells. Clonogenic cells from LTC were enumerated at the end of the fifth week. There was a significantly increased formation of CFU from test cells than from control cells, indicating better preservation of early progenitors in test cells. Our results suggest that use of a combination of catalase and trehalose as a supplement in the conventional freezing medium results in better protection of growth factor receptors, adhesion molecules, and functionality of hematopoietic cells, yielding a better graft quality.

Transplantable stem cells: home to specific niches

PURPOSE OF REVIEW

Although the concept of engraftment and clinical reconstitution of the bone marrow was described several decades ago, the analysis of individual steps within this process remains a major focus of much current research in stem cell biology. In particular, this extends to the identification and characterization of the specific stem cell niche first proposed by Schofield in 1978. It is appropriate, therefore, that on the 25th anniversary of this publication, that we review recent progress in our understanding of the location and composition of the bone marrow stem cell niche and of the mechanisms involved in the initial phases of hematopoietic stem cell engraftment.

RECENT FINDINGS

During the past 12 months there have been significant advancements in our understanding of the interplay of molecules involved in the homing of hematopoietic stem cells to the bone marrow. In addition, innovative methodologies have become available for the visualization of hematopoietic stem cells within the bone marrow in situ. In an important development in this area, studies our now focusing on events after transendothelial migration into the marrow cords, including mechanisms involved in hematopoietic stem cell migration to and lodgment within the hematopoietic stem cell niche. Furthermore, there have been numerous new reports analyzing the molecular regulation of hematopoietic stem cells within the bone marrow niche in situ.

SUMMARY

Overall, recent advancements in our understanding of hematopoietic stem cell biology and, in particular, the interaction of hematopoietic stem cells with the hematopoietic microenvironment paves the way for expanded use in regenerative medicine.

N-linked sialyated sugar receptors support haematopoietic cell-osteoblast adhesions

Haematopoietic progenitor cells proliferate and develop predominantly when they adhere to bone marrow stromal cells such as osteoblasts. Therefore, changes in adhesion may be a common mechanism by which stem cells survive, mature and properly traffic between the bone marrow and the circulation. To characterize these adhesion molecules, we reduced the bone marrow cavity to a simple adhesion assay between KG1a (a CD34+ haematopoietic cell line) and osteosarcoma monolayers (MG-63 or SaOS-2). The data demonstrated that adhesion was mediated by cell-to-cell rather than cell-to-matrix contact, was sensitive to trypsin, calcium chelators and glycosylation inhibitors. Selective pretreatment attributed the constitutive binding to N-linked glycans on KG1a. When carboprocessing was inhibited later at the high mannose intermediate (via deoxymannojirimycin), adhesion was retained. Surprisingly, binding of KG1a to SaOS-2 increased past constitutive levels as doses of tunicamycin or deoxymannojirimycin dropped. Selective pretreatment suggested that this 'inducible' binding resides with molecule(s) on SaOS-2. If the terminal sialic acid was digested (via neuraminidase), this induced response was duplicated. These data, verified in primary cells, suggest that the initial tethering between blood and bone cells in this model is probably due to heavily glycosylated, rapidly processed protein(s) on both cell types.

Nonexpanded primary lung and bone marrow–derived mesenchymal cells promote the engraftment of umbilical cord blood–derived CD34+ cells in NOD/SCID mice

OBJECTIVE

Previously, we have found that human culture-expanded fetal lung-derived mesenchymal stem cells (MSC) promote the engraftment of umbilical cord blood (UCB)-derived CD34((+)) cells. The high frequency of MSC in fetal lung allowed us to study whether this represented a biological feature of these cells or a property that was acquired during expansion in culture.

MATERIALS AND METHODS

Irradiated NOD/SCID mice (n=80) were transplanted with 0.1x10(6) UCB CD34(+) cells in the presence or absence of 10(6) primary nonexpanded or culture-expanded fetal lung, liver, or BM CD45(-) cells, or with nonexpanded fetal lung liver or BM CD45(-) cells only.

RESULTS

In comparison with transplantation of UCB CD34(+) cells only, cotransplantation of UCB CD34(+) cells and primary fetal lung or BM CD45(-) cells resulted in a significantly higher level of engraftment (% hCD45(+) cells) in BM, PB, and spleen. In addition, primary mesenchymal cells derived from adult BM had a similar promoting effect. The engraftment-enhancing effect was similar to that of culture-expanded fetal lung and BM MSC. Primary mesenchymal cells, but not culture-expanded MSC, were detected in recipient mice, suggesting that the primary cells were able to home and that this capacity was lost after expansion.

CONCLUSIONS

These results show that primary mesenchymal cells from fetal lung and BM promote the engraftment of UCB-derived CD34(+) cells to a similar degree as culture-expanded MSC, indicating that it reflects a biological property of primary MSC that is preserved during expansion in culture.

Adhesion of MNC to extracellular matrix proteins following in vitro photochemotherapy

BACKGROUND

Photochemotherapy is a safe and effective treatment for patients with drug-resistant severe GvHD. The technique involves the exposure of MNC to psoralen and UVA light (PUVA). We have investigated the effect of in vitro PUVA on MNC adhesion to extracellular matrix (ECM) proteins.

METHODS

MNC were isolated from peripheral blood (PB) and umbilical cord blood (UCB), and treated with PUVA. After labeling by a chemiluminescent probe, MNC were plated on ECM proteins (collagen, fibronectin, vitronectin and laminin) and the number of adherent cells was measured.

RESULTS

Untreated MNC from both PB and UCB showed a similar adhesion to the substrates. As a consequence of exposure to PUVA, most of PB samples showed significantly enhanced adhesion to the ECM proteins; on the other hand, UCB-recovered MNC did not significantly modify their adhesion.

DISCUSSION

MNC adhesion to ECM components is mediated by integrins, a family of cell membrane receptors; the ligand-binding affinity of certain integrins may be modulated by different stimuli. PUVA treatment of PB-recovered MNC may induce the up-regulation of the ligand-binding affinity of the integrins involved in the adhesion to ECM proteins. The finding of unmodified UCB cell adhesion after PUVA, may be related to the functional immaturity of lymphocytes at birth.

W/Wv marrow stromal cells engraft and enhance early erythropoietic progenitors in unconditioned Sl/Sld murine recipients

Transplantation of marrow stromal cells may provide a means of modulating hematopoiesis and serve as a form of cell therapy. We employed a murine transplant model using Sl/Sl(d) mice, which have macrocytic anemia due to defective expression of stem cell factor (SCF) on bone marrow stromal cells. Donor cells were derived from the complementary mutant strain W/W(v), which also exhibit anemia, due to mutations in c-kit, the SCF receptor expressed on hematopoietic stem cells. The strength of this model is that any correction of the Sl/Sl(d) anemia from the infusion of W/W(v) stromal cells can be attributed to the effect of the stromal cells and not to contaminating W/W(v) hematopoietic stem cells, a major concern in experiments involving wild-type animals. Cultured stromal cells were infused into unconditioned non-splenectomized Sl/Sl(d) mice. Engraftment of donor stromal cells reached levels of up to 1.0% of total marrow cells 4 months post transplant. However, stromal engraftment was not detectable in the spleen. Recipients of W/W(v) stroma showed a significant increase in the committed erythroid progenitors compared with those receiving Sl/Sl(d) stromal cells: 109 +/- 26 vs 68 +/- 5 CFU-E per 10(5) BMC, P = 0.002; 25 +/- 10 vs 15 +/- 5 BFU-E per 10(5) BMC, P = 0.037, for W/W(v) and Sl/Sl(d) stroma recipients, respectively. Despite this increase in erythroid progenitors, the anemia was not corrected. Our data suggest that in this murine model, splenic erythropoiesis may influence stromal cell therapy, and that higher levels of marrow engraftment may be necessary to obtain a clinically significant effect.

P-selectin, and not E-selectin, negatively regulates murine megakaryocytopoiesis

To assess the role of P-selectin and E-selectin in megakaryocytopoiesis, in vitro assays were performed in animal models deficient in both adhesion receptors. There was a significantly greater number of IL-3-responsive megakaryocyte progenitors CFU (CFU-MK) and an increase in immature megakaryoblasts in response to IL-6 in the P-selectin-null mice compared with the wild-type controls. Furthermore, P-selectin-null mice showed a greater number of CFU-MK colonies derived from CD34(+) cells in response to IL-3 or IL-3 plus stem cell factor. A significant shift in baseline ploidy with a reduction in 8N cells and an increase in 32N cells was also observed in the P-selectin-null mice. Secretion of the inhibitory growth factor TGF-beta1 and not TGF-beta2 was significantly lower in the supernatants of cultures containing bone marrow cells from P-selectin-deficient mice as compared with those from the wild-type control bone marrow cells. No differences in the responsiveness of murine CFU-MK, immature megakaryocytes, or 5-fluorouracil-selected stem cells to cytokines were observed in E-selectin-null mice as compared with the control mice. These studies indicate that the absence of P-selectin, and not E-selectin, resulted in an altered adhesion environment with subsequent expansion of megakaryocyte progenitors and immature megakaryoblasts, enhanced secretion of TGF-beta1, and apparent increased responsiveness to inflammatory cytokines.

Enhancement of G-CSF-induced stem cell mobilization by antibodies against the beta 2 integrins LFA-1 and Mac-1

The beta 2 integrins leukocyte function antigen-1 (LFA-1, CD11a) and macrophage antigen-1 (Mac-1, CD11b) have been reported to play a role in the attachment of CD34(+) cells to stromal cells in the bone marrow. When administered prior to interleukin-8 (IL-8), anti-LFA-1 antibodies completely prevent the IL-8-induced mobilization of hematopoietic stem cells in mice. Here, we studied the role of anti-beta 2 integrin antibodies in granulocyte colony-stimulating factor (G-CSF)-induced mobilization of hematopoietic progenitor cells. Administration of antibodies against the alpha chain of LFA-1 or against the alpha chain of Mac-1 followed by daily injections of G-CSF for more than 1 day resulted in a significant enhancement of mobilization of hematopoietic progenitor cells when compared with mobilization induced by G-CSF alone. Also, the number of late (day 28) cobblestone area-forming cells in vitro was significantly higher after mobilization with anti-LFA-1 antibodies followed by 5 microg G-CSF for 5 days than with G-CSF alone (119 +/- 34 days vs 17 +/- 14 days), indicating mobilization of repopulating stem cells. Pretreatment with blocking antibodies to intercellular adhesion molecule-1 (ICAM-1; CD54), a ligand of LFA-1 and Mac-1, did not result in an effect on G-CSF-induced mobilization, suggesting that the enhancing effect required an interaction of the beta 2 integrins and one of their other ligands. Enhancement of mobilization was not observed in LFA-1-deficient (CD11a) mice, indicating that activated cells expressing LFA-1 mediate the synergistic effect, rather than LFA-1-mediated adhesion.

Application of peripheral blood stem cells (PBSC) mobilized by recombinant human granulocyte colony stimulating factor for allogeneic PBSC transplantation and the comparison of allogeneic PBSC transplantation and bone marrow transplantation

Recombinant human granulocyte colony stimulating factor (rhG-CSF)-mobilized peripheral blood stem cells (PBSC) are now widely used for allogeneic PBSC transplantation (alloPBSCT). Large numbers of hematopoietic progenitor cells mobilized by rhG-CSF would be considered equivalent or better than bone marrow (BM) cells and would be used as an alternative to BM for allogeneic hematopoietic stem cell transplantation. The complications associated with the administration of rhG-CSF and apheresis in PBSC collection in formal donors are well tolerated and usually acceptable in the short term but some hazardous adverse events such as splenic rupture and cardiac arrest are reported although the incidence is very low. Protective means and stopping rules for safe donation in the collection of PBSC are established. The characteristics of PBSC were clarified; the expression of some adhesion molecules such as CD49d on CD34 positive cells of PBSC have been shown to be low compared to BM stem cells. In alloPBSCT compared with allogeneic BM transplantation (alloBMT), the incidence and frequency of graft versus host disease (GVHD) is of concern because high number of T lymphocytes are infused in alloPBSCT. The incidence and severity of acute GVHD are not increased but chronic GVHD is higher in alloPBSCT compared with alloBMT. The outcome of alloPBSCT and BMT are almost equivalent and conclusive results regarding survival are not yet available.

Homing, cell cycle kinetics and fate of transplanted hematopoietic stem cells

Homing of transplanted hematopoietic stem cells to recipient bone marrow is a critical step in engraftment and initiation of marrow reconstitution. At present, only partial understanding of the cellular and molecular mechanisms governing homing exists. Likewise, only an incomplete list of adhesion molecules implicated in directing the trafficking of stem cells to the marrow microenvironment is available. Opposing hypotheses that attribute homing to an orderly and orchestrated cascade of events or to random migration of circulating cells find ample experimental support. Also unsettled is the fate of marrow-homed cells shortly after transplantation and the rapidity at which they begin to proliferate in their new marrow microenvironment. The limited number of studies in this field and disparities in their experimental design intensifies the confusion surrounding these critical aspects of stem cell biology. However, this area of research is moving forward rapidly and results capable of clarifying many of these issues are forthcoming.

An in vitro model of hematopoietic stem cell homing demonstrates rapid homing and maintenance of engraftable stem cells

Hematopoietic stem cell (HSC) homing is believed to rely heavily on adhesion interactions between stem cells and stroma. An in vitro assay was developed for adhesion of engraftable HSCs in bone marrow suspensions to pre-established Dexter-type long-term bone marrow culture stromal layers. The cell numbers in the adherent layer and supernatant were examined, along with the engraftment capability of adherent layer cells to indicate the number of HSCs that homed to in vitro stroma. The cell number in the supernatant declined over the 24-hour period. The number of test cells adhering to the stromal layer increased during the first hour and then fell at 6 and 24 hours. The number of test HSCs adhering to the stromal layer was substantial at 20 minutes, increased during the first hour, and then remained constant at 1, 6, and 24 hours of adhesion. These data indicate that adhesion of engraftable HSCs occurs quickly and increases during the first hour of contact with pre-established stroma, that adhesion plateaus within 1 hour of contact, and that HSCs maintain their engraftment capability for at least 24 hours of stromal adhesion. Long-term engraftment from test cells at more than 1 hour of adhesion represents 70.7% of the predicted engraftment from equivalent numbers of unmanipulated marrow cells, indicating that 2 of 3 test engraftable HSCs adhered. These findings demonstrate the usefulness of this model system for studying stem-stromal adhesion, allowing further dissection of the mechanism of HSC homing and exploration of possible manipulations of the process. (Blood. 2001;98:1012-1018)

The Hematopoietic Microenvironment: Phylogeny and Ontogeny of the Hematopoietic Microenvironment

Although there is no formalized area of study called phylogeny or ontogeny of the hematopoietic microenvironment, new models and molecular tools are now available for such studies. The concept of a hematopoietic microenvironment has developed from the need to answer basic questions about migration, control of proliferation and differentiation of lymphohematopoietic cells; e.g. how are cells with the same genes induced to express different sets of these genes which lead to differentiation. These questions were first approached when cells could only be identified morphologically. The ontogeny of hematopoiesis was traced from the blood islands of the embryonic yolk sac, to the fetal liver, spleen, and bone marrow. Cells with reticular morphology were associated with areas of hematopoiesis and, in the embryo, they were thought to give rise to both hematopoietic and supportive cells. In the 1960's the classic work of McCulloch, Till and Siminovitch led the study of hematopoietic precursors which have no distinctive morphological identity and are too infrequent to study microscopically. These cells were identified by their functions; e.g. colony formation in culture in the presence of certain factors, production of spleen colonies or rescue of lethally irradiated mice. Cells with these functions were also found sequentially in the yolk sac blood islands, in the aorta/mesonephros, fetal liver, spleen, and bone marrow during development. The question remained, what regulates the proliferation and differentiation of these cells and why do they home to different sites in different stages of development? Among the laboratories studying spleen colonies, a controversy arose as to whether differentiation decisions were stochastic or induced by extra cellular factors. Dexter and Greenberger developed the long-term bone marrow culture system which has aided in studying the roles of factors such as cell-cell contact and extracellular matrix in hematopoietic differentiation. The molecular identification of ligand/receptor pairs such as ckit and KL as well as transactivating factors that control whole sets of lineage related genes such as the GATAs and Ikaros, may lead to clarification of the stochastic versus induced differentiation issue. Chimeric bird and frog embryos and analysis of mutations effecting hematopoiesis in frogs and zebrafish have helped to trace the earliest hematopoietic development in the embryo and to determine what influences it. The identification of genes that alter development of hematopoiesis opens the possibility of comparing microenvironmental control mechanisms in various present day organisms and relating these to evolutionary events. Many basic questions relevant to the interaction between hematopoietic cells and their microenvironment can be addressed by studying "simple" organisms in which the answers may be more easily determined than in mice or humans. Examples of possibly useful organisms, range from the teliosts such as zebrafish to algae such as Volvox, a two cell organism, to Dictyostelium which change from 1 to many cell types and in the process, migrate, adhere and differentiate.

Mucin-like molecules as modulators of the survival and proliferation of primitive hematopoietic cells

Current data suggest that interplay between two classes of molecules contributes to the regulation of hematopoiesis: hematopoietic growth factors, which regulate the survival, proliferation, and development of primitive hematopoietic cells and cell adhesion molecules (CAMs), which are responsible for the localization of hematopoiesis to the bone marrow (BM) and for mediating physical association between developing hematopoietic cells and marrow stromal tissue. A range of cell surface molecules representing several CAM superfamilies including integrins, selectins, the immunoglobulin gene superfamily and an emerging family of mucin-like molecules (the sialomucins) are involved in supporting cell-cell and cell-extracellular matrix (ECM) interactions between primitive hematopoietic cells and the stromal cell-mediated hematopoietic microenvironment (HM) of the bone marrow. There is abundant evidence in non-hematopoietic tissues that CAMs are signalling molecules which participate in a range of signal transduction events important not only for regulating cell adhesion and motility, but also for cell growth and survival. Although the signalling functions of CAMs have not been studied extensively in primitive hematopoietic progenitors (HPCs), extrapolation from burgeoning data in other systems is consistent with the hypothesis that hematopoiesis within the BM is regulated by interaction between signals generated locally by CAMs and those elicited by cytokines. Evidence in support of this notion was initially provided by studies on normal HPCs demonstrating cross-talk between members of the integrin superfamily and cytokine receptors. In this article we review recent reports that mucin-like molecules are also signalling molecules on primitive hematopoietic cells and that the signals they deliver potently inhibit hematopoiesis.

Adhesion molecule expression, clinical features and therapy outcome in childhood acute lymphoblastic leukemia

In view of the relevance of adhesion molecule expression for the mechanisms of homing, trafficking and spreading of malignant cells, we have investigated the expression of surface adhesion molecules in lymphoblasts from 57 acute lymphoblastic leukemia (ALL) cases and tried to correlate the adhesive phenotype with immunological typing, prognostic factors at diagnosis and clinical follow-up. Blasts from all cases expressed adhesion molecules at high rates. Beta1 integrin chain (CD18) was consistently found on blasts from most ALL cases: among integrins of the beta2 family. LFA-1 was detected in 58% of cases, in the virtual absence of other alpha chains. CD54 and CD58 were expressed in variable proportions by ALL blasts and CD44 was detected in the majority of the malignant cells, whereas the CD62L selectin was only present in 24% of cases. B-lineage ALL's displayed similar adhesion molecule phenotypes irrespective of maturational stages of the leukemic cells. We found a significantly reduced expression of beta2 alphaL integrins in the hybrid ALL cases (CD13 and/or CD33 positive). However, these cases did not show differences in clinical presentation and behaviour in comparison with patients of other groups. We did not find a significant correlation between adhesion molecule expression and well established risk factors (age, white blood cell count, central nervous system involvement, chromosomal abnormalities), with the exception of splenomegaly, that was significantly associated with CD18 expression. In the follow-up, no evidence of significant correlation between adhesive phenotype and adverse events such as leukemic relapse and death was found. In conclusion, although expression of adhesion molecules on lymphoblasts confirms the phenotypic heterogeneity of ALL, it appears that this is not relevant for the clinical aspects of the disease and for prognosis.

Integrin-mediated interactions between human bone marrow stromal precursor cells and the extracellular matrix

To date, the precise interactions between bone marrow stromal cells and the extracellular matrix that govern stromal cell development remain unclear. The integrin super-family of cell-surface adhesion molecules represents a major pathway used by virtually all cell types to interact with different extracellular matrix components. In this study, purified populations of stromal precursor cells were isolated from the STRO-1-positive fraction of normal human marrow, by fluoresence-activated cell sorting, and then assayed for their ability to initiate clonogenic growth in the presence of various integrin ligands. Bone marrow-derived stromal progenitors displayed differential growth to fibronectin, vitronectin, and laminin, over collagen types I and III, but showed a similar affinity for collagen type IV. The integrin heterodimers alpha1beta1, alpha2beta1, alpha5beta1, alpha6beta1, alpha(v)beta3, and alpha(v)beta5 were found to coexpress with the STRO-1 antigen on the cell surface of CFU-F, using dual-color analysis. Furthermore, only a proportion of stromal precursors expressed the integrin alpha4beta1, while no measurable levels of the integrin alpha3beta1 could be detected. Subsequent adhesion studies using functional blocking antibodies to different integrin alpha/beta heterodimers showed that stromal cell growth on collagen, laminin, and fibronectin was mediated by multiple beta1 integrins. In contrast, cloning efficiency in the presence of vitronectin was mediated in part by alpha(v)beta3. When human marrow stromal cells were cultured under osteoinductive conditions, their ability to form a mineralized matrix in vitro was significantly diminished in the presence of a functional blocking monoclonal antibody to the beta1 integrin subunit. The results of this study indicate that beta1 integrins appear to be the predominant adhesion receptor subfamily utilized by stromal precursor cells to adhere and proliferate utilizing matrix glycoproteins commonly found in the bone marrow microenvironment and bone surfaces. Furthermore, these data suggest a possible role for the beta1 integrin subfamily during the development of stromal precursor cells into functional osteoblast-like cells.

Decrease of L-selectin expression on human CD34+ cells on freeze-thawing and rapid recovery with short-term incubation

In the homing process of hematopoietic progenitor cells (HPCs) to bone marrow, several adhesion molecules play important roles. However, HPCs are subjected to dramatic alteration of freeze-thawing that could affect these molecules. In this study, we investigated the effect of cryopreservation on the expression of adhesion molecules on HPCs. The expression of different adhesion molecules on CD34+ cells was examined by flow cytometry using an immunofluorescence technique. Changes in expression before and after cryopreservation, and that of L-selectin on addition of dimethylsulfoxide (DMSO) or with serum incubation, were investigated. The relationship between expression and function was also determined using a transmigration assay system. L-selectin expression on CD34+ cells derived from mobilized peripheral blood (MPB), bone marrow (BM), and umbilical cord blood (CB) was significantly decreased from 37% to 23%, 48% to 11%, and 67% to 19%, respectively, by the freeze-thawing process, while the expression of other adhesion molecules was not appreciably changed. Within 30 minutes of incubation with DMSO, L-selectin expression on CD34(+) cells significantly decreased from 65% to 22%. Furthermore, this was completely prevented by the matrix metalloproteinase (MMP) inhibitor, KB-R8301, indicating that the loss of L-selectin was due to shedding mediated by an MMP. To determine if L-selectin expression can be upregulated after cryopreservation, thawed samples were cultured overnight with serum. Values were observed to return to or rise higher than those of the fresh samples, this being particularly rapid and pronounced when the CD34+ cells were cocultured with the human BM stromal cell line, HS-27A. Therefore, this adhesion molecule could possibly be restored in vivo after transplantation in a way similar to the in vitro case. Despite considerable damage to HPCs during cryopreservation, changes in these cells are reversible, in line with successful restoration of hematopoiesis after transplantation.

A hematopoietic cell L-selectin ligand that is distinct from PSGL-1 and displays N-glycan–dependent binding activity

Human hematopoietic progenitor cells express L-selectin and also express PSGL-1, a ligand for all selectins. Using a shear-based adhesion assay, a hematopoietic cell L-selectin ligand (HCLL) that is expressed on the hematopoietic cell line KG1a and on normal human hematopoietic progenitors was previously identified. To characterize the structural biology of HCLL and to define its relationship to PSGL-1, the effects of chemical and enzymatic treatments on HCLL activity of KG1a cells and membrane preparations were analyzed. Protease digestions and chemical treatments of KG1a cells and membranes indicated that HCLL is an integral membrane glycoprotein. Glycosidase digestions of membrane protein preparations and metabolic treatments of KG1a cells with glycosylation processing modifiers revealed that L-selectin binding determinants on HCLL are sialofucosylated structures presented on complex-type N-glycans. Adhesion assays and biochemical studies showed that this glycoprotein is also expressed on circulating blasts in native acute leukemias. HCLL is distinguishable from PSGL-1: (1) KG1a cells sorted for PSGL-1 expression had equivalent HCLL activity; (2) anti–PSGL-1 blocking antibodies and proteases known to eliminate L-selectin binding to PSGL-1 had no effect on HCLL binding activity of KG1a cells; (3) blasts from native leukemias with low expression of PSGL-1 and CD34 display high HCLL activity; and (4) despite high level expression of PSGL-1, HCLL activity was absent on HL60 cells. These data provide first evidence of a naturally expressed membrane L-selectin ligand expressing binding determinant(s) on an N-linked glycoconjugate. This novel ligand may help mediate L-selectin–dependent cell-cell adhesive interactions within the cytoarchitecture of the bone marrow microenvironment.

A hematopoietic cell L-selectin ligand that is distinct from PSGL-1 and displays N-glycan–dependent binding activity

Human hematopoietic progenitor cells express L-selectin and also express PSGL-1, a ligand for all selectins. Using a shear-based adhesion assay, a hematopoietic cell L-selectin ligand (HCLL) that is expressed on the hematopoietic cell line KG1a and on normal human hematopoietic progenitors was previously identified. To characterize the structural biology of HCLL and to define its relationship to PSGL-1, the effects of chemical and enzymatic treatments on HCLL activity of KG1a cells and membrane preparations were analyzed. Protease digestions and chemical treatments of KG1a cells and membranes indicated that HCLL is an integral membrane glycoprotein. Glycosidase digestions of membrane protein preparations and metabolic treatments of KG1a cells with glycosylation processing modifiers revealed that L-selectin binding determinants on HCLL are sialofucosylated structures presented on complex-type N-glycans. Adhesion assays and biochemical studies showed that this glycoprotein is also expressed on circulating blasts in native acute leukemias. HCLL is distinguishable from PSGL-1: (1) KG1a cells sorted for PSGL-1 expression had equivalent HCLL activity; (2) anti–PSGL-1 blocking antibodies and proteases known to eliminate L-selectin binding to PSGL-1 had no effect on HCLL binding activity of KG1a cells; (3) blasts from native leukemias with low expression of PSGL-1 and CD34 display high HCLL activity; and (4) despite high level expression of PSGL-1, HCLL activity was absent on HL60 cells. These data provide first evidence of a naturally expressed membrane L-selectin ligand expressing binding determinant(s) on an N-linked glycoconjugate. This novel ligand may help mediate L-selectin–dependent cell-cell adhesive interactions within the cytoarchitecture of the bone marrow microenvironment.

Reduction of marrow hematopoietic progenitor and stem cell content is not sufficient for enhanced syngeneic engraftment

The mechanisms regulating long-term engraftment of primitive stem cells are largely unknown. Most conditioning strategies use myeloablative agents for experimental or clinical hematopoietic stem cell transplantation. Host conditioning regimens, in part, have been designed on the assumption that transplanted cells home to specific marrow sites and if these sites are occupied by host stem cells, engraftment will not take place. However, there is now evidence that stable long-term syngeneic engraftment may occur in the absence of host marrow stem cell depletion. To further study the association of engraftment with stem cell depletion, we investigated whether the marked egress of hematopoietic progenitor and stem cells from the marrow into the peripheral blood in C57BL6 mice following a single dose of cyclophosphamide (day 1) and four days of G-CSF (days 3-6) afforded an increased opportunity for long-term syngeneic donor engraftment. During and after mobilization, glucose phosphate isomerase (GPI)-1(b) mice received 30 x 10(6) GPI-1(a) marrow cells without further myeloablation. The level of donor/recipient chimerism was assessed in cell lysates after six months. Increased long-term syngeneic donor engraftment was observed prior to mobilization (before day 6), during a period of active hematopoietic regeneration following the administration of cyclophosphamide. Hematopoietic regeneration was evidenced by a reduced but rapidly increasing marrow cellularity and an increased proportion of hematopoietic progenitors in S-phase. In contrast, long-term syngeneic donor engraftment was not increased over controls during the period of maximum progenitor and stem cell mobilization (after day 5). At this time there were minimal numbers of progenitor and stem cells in the marrow. These data suggest that in the absence of host stem cell ablation, maximal engraftment does not occur during marrow progenitor or stem cell depletion, suggesting that the presence of "open" marrow sites is not a prerequisite for engraftment. The mechanisms for increased engraftment during progenitor cell regeneration following cyclophosphamide need further investigation. Understanding the mechanisms for engraftment without host stem cell ablation may allow strategies for improved long-term engraftment of syngeneic or autologous stem cells with reduced post-transplant toxicity.

Effects of the chemokine stromal cell-derived factor-1 on the migration and localization of precursor-B acute lymphoblastic leukemia cells within bone marrow stromal layers

Acute lymphoblastic leukemia (ALL) blasts undergo migration into layers of bone marrow fibroblasts (BMF) in vitro, utilizing the beta1 integrins VLA-4 and VL-5 as adhesion molecules. However, it has been unclear as to whether this is a selective process mediated by specific chemoattractant molecules, or simply a reflection of the highly motile nature of early B cell precursors. We further characterized this process using a transwell culture system, in which the two chambers were separated by an 8 microm diameter microporous membrane, through which leukemic cells could move. When a BMF layer was grown on the upper surface of the membrane there was an 84.1% reduction in transmigration of the human pre-B ALL cell line NALM-6 into the lower chamber, compared to control membrane with no BMF layer. Localization of leukemic cells under the BMF layer was confirmed ultrastructurally, suggesting the possibility that the migration of leukemic cells was directed by a chemotactic agent secreted by BMF. The involvement of the chemokine stromal cell-derived factor-1 (SDF-1) in this process was next investigated. BMF were shown to express m-RNA for SDF-1. Addition of SDF-1 at 100 ng/ml into the lower chamber increased transmigration of NALM-6 across the membrane by 2.2-fold, and also induced a 1.4- to 6.1-fold increase in movement of NALM-6 through a BMF layer into the lower chamber. The receptor for SDF-1, CXCR4, was demonstrated by flow cytometry on all 10 cases of precursor-B ALL analyzed, as well as on NALM-6, KM-3 and REH lines. An inhibitory antibody to CXCR4 was able to block the migration of NALM-6 cells into BMF monolayers grown on plastic by 51%, and in nine cases of ALL by 8-40%, as well as partially inhibit transmigration of leukemic cells through BMF layers along an SDF-1 concentration gradient. These results confirm that precursor-B ALL cells selectively localize within bone marrow stroma in vitro, and that this process is partially due to the stromal chemokine SDF-1 binding to its receptor CXCR4 on leukemic cells. SDF-1 may be important in influencing the localization of precursor-B ALL cells in marrow microenvironmental inches which regulate their survival and proliferation.

Therapeutic relevance of CD34 cell dose in blood cell transplantation for cancer therapy

PURPOSE

To review recent advances in peripheral-blood progenitor-cell (PBPC) transplantation in order to define the optimal cell dose required for autologous and allogeneic transplantation.

MATERIALS AND METHODS

A search of MEDLINE was conducted to identify relevant publications. Their bibliographies were also used to identify further articles and abstracts for critical review.

RESULTS

The CD34(+) cell content of a graft is regarded as an accurate predictor of engraftment success. Postchemotherapy autologous PBPC transplantation with >/= 5 x 10(6) CD34(+) cells/kg body weight leads to more rapid engraftment than does transplantation of lower cell doses. Further increases in transplant cell dose further accelerate platelet but not neutrophil engraftment. Evidence that long-term hematopoietic recovery may be more accurately predicted by the subpopulation of primitive progenitors transplanted suggests that the content of CD34(+)CD33(-) and long-term culture-initiating cells in cell collection samples may be important for predicting successful engraftment, particularly in patients with poor mobilization. Allogeneic transplantation has been limited by concerns regarding graft-versus-host disease and the use of hematopoietic growth factors in donors. The risk of graft rejection and engraftment failure after HLA-mismatched allogeneic transplantation may be overcome by intensive chemoradiotherapy and the infusion of large numbers of T cell-depleted hematopoietic stem cells.

CONCLUSION

An optimal cell dose of >/= 8 x 10(6) CD34(+) cells/kg seems to be recommended for autologous PBPC transplantation. This dose facilitates the administration of scheduled chemotherapy on time and reduces the demand for other supportive therapies. A combination of growth factors may enable patients with poor mobilization to achieve a collection sufficient to allow transplantation. The optimum PBPC dose for allogeneic transplantation remains to be defined.

Peripheral blood progenitor cell mobilization in healthy donors receiving recombinant human granulocyte colony-stimulating factor

OBJECTIVE

We analyzed the incidence of primitive (LTC-IC) and committed (CFU-mix, BFU-E, CFU-GM) hematopoietic progenitors detected under steady-state conditions and upon progenitor cell mobilization in a cohort of healthy donors receiving recombinant human granulocyte colony-stimulating factor (rhG-CSF).

MATERIALS AND METHODS

Healthy donors (n = 30) of HLA-mismatched or -matched stem cell transplants were mobilized with rhG-CSF (8 microg/Kg body weight subcutaneously twice daily until completion of leukapheresis). PBPC collections were started after 4 days of rhG-CSF therapy.

RESULTS

Steady-state incidence of bone marrow LTC-IC, but not committed progenitors, significantly correlated with the numbers of mobilized CD34+ cells (r = 0.6, p = 0.004), CFU-GM (r = 0.79, p = 0.0005) and CFC (r = 0.76, p = 0.001) detected after 4 days of rhG-CSF therapy. Statistically significant correlations were also found between steady-state blood CFU-GM and peak numbers of CD341 cells (r = 0.68, p = 0.001), numbers of day 4 CD341 cells (r = 0.52, p = 0.005), CFU-GM (r = 0.63, p = 0.002), and CFC (r = 0.61, p = 0.003).

CONCLUSION

Our data show that in normal volunteers baseline marrow LTC-IC and blood CFU-GM correlate with rhG-CSF-mobilized PBPC. The potential clinical relevance of these findings in the identification of poor mobilizers will be tested in a prospective study.

Cotransplantation of stroma results in enhancement of engraftment and early expression of donor hematopoietic stem cells in utero

Although promising, clinical and experimental efforts at in utero hematopoietic stem cell (HSC) transplantation currently are limited by minimal donor cell engraftment and lack of early donor cell expression after transplantation. We reasoned that cotransplantation of stromal elements (ST) might condition the fetal microenvironment for the engraftment of donor HSC and facilitate precocious bone marrow (BM) hematopoiesis. In this study we cotransplanted sheep ST, derived from adult or fetal BM, with either adult or fetal HSC, into preimmune fetal sheep. We analyzed donor cell chimerism in BM and peripheral blood and compared levels of chimerism achieved with recipients of HSC alone. In all experimental groups, stromal cotransplantation markedly increased the level of peripheral blood donor cell expression at 60 days after transplantation relative to controls. Adult BM-derived stroma cotransplanted with adult HSC provided the highest levels of circulating donor cells, whereas fetal-derived stroma was less effective. In addition, ST cotransplantation resulted in increased donor cell engraftment in the BM and led to significantly increased levels of donor hematopoiesis for over 30 months after transplant. Cotransplantation of stroma may represent a valuable clinical strategy for optimal application of in utero HSC transplantation.

Expression of the adhesion molecules CD49d and CD49e on G-CSF-mobilized CD34+ cells of patients with solid tumors or non-Hodgkin's and Hodgkin's lymphoma and of healthy donors is inversely correlated with the amount of mobilized CD34+ cells

The yield of CD34+ PBPC and colony-forming units-granulocyte-macrophage (CFU-GM) in leukapheresis products and the expression of the adhesion molecules CD11a, CD31, CD49d, CD49e, CD54, CD58, CD62L, c-kit (CD117), Thy-1 (CD90), CD33, CD38, and HLA-DR on CD34+ PBPC were analyzed in patients with cancer of the testis (n = 10), breast cancer (n = 10), Hodgkin's disease (n = 20), high-grade (n = 20) and low-grade (n = 20) non-Hodgkin's lymphoma, and healthy donors (n = 20) undergoing G-CSF (filgrastim)-stimulated PBPC mobilization. For each disease entity, G-CSF was administered in two different doses, 10 microg G-CSF/kg body weight (BW)/day s.c. vs. 24 microg G-CSF/kg BW s.c./day in steady-state condition. Data were compared for each dose group separately. Patients with cancer of the testis and breast cancer mobilized significantly more CD34+ cells than patients with high-grade and low-grade non-Hodgkin's lymphoma and Hodgkin's disease (p<0.05). Correspondingly, expression of CD49d on CD34+ PBPC was significantly lower in the same patients with cancer of the testis compared with high-grade and low-grade non-Hodgkin's lymphoma and Hodgkins' disease and in patients with breast cancer compared with high-grade and low-grade non-Hodgkin's lymphoma, Hodgkins's disease, and healthy donors. Similar results were obtained for CD49e. These data suggest that the expression of the adhesion molecules CD49d and CD49e on G-CSF-mobilized CD34+ cells of patients with solid tumors, non-Hodgkin's lymphoma, Hodgkin's disease, and healthy donors is inversely correlated with the amount of mobilized CD34+ cells.

Prevention of interleukin-8-induced mobilization of hematopoietic progenitor cells in rhesus monkeys by inhibitory antibodies against the metalloproteinase gelatinase B (MMP-9)

Previously, we demonstrated that IL-8 induces rapid mobilization of hematopoietic progenitor cells (HPC) from the bone marrow of rhesus monkeys. Because activation of neutrophils by IL-8 induces the release of gelatinase B (MMP-9), which is involved in the degradation of extracellular matrix molecules, we hypothesized that MMP-9 release might induce stem cell mobilization by cleaving matrix molecules to which stem cells are attached. Rhesus monkeys were treated with a single i.v. injection of 0.1 mg/kg human IL-8, which resulted in a 10- to 100-fold increase in HPC within 30 min after injection. Zymographic analysis revealed a dramatic instantaneous increase in the plasma levels of MMP-9, followed by the increase in circulating HPC. Enzyme levels decreased at 2 h after injection of IL-8, simultaneously with the decrease in the numbers of circulating HPC. To test the hypothesis that MMP-9 induction was involved in HPC mobilization, rhesus monkeys were treated with a highly specific inhibitory monoclonal anti-gelatinase B antibody. Anti-gelatinase B at a dose of 1-2 mg/kg completely prevented the IL-8-induced mobilization of HPC, whereas a dose of 0.1 mg/kg had only a limited effect. Preinjection of inhibitory antibodies did not preclude the IL-8-induced production and secretion of MMP-9. Pretreatment with an irrelevant control antibody did not affect IL-8-induced mobilization, showing that the inhibition by the anti-gelatinase B antibody was specific. In summary, IL-8 induces the rapid systemic release of MMP-9 with concurrent mobilization of HPC that is prevented by pretreatment with an inhibitory anti-gelatinase B antibody, indicating that MMP-9 is involved as a mediator of the IL-8-induced mobilization of HPC.

Different expression of adhesion molecules on CD34+ cells in AML and B-lineage ALL and their normal bone marrow counterparts

The expression of adhesion molecules on CD34+ cells in acute myeloid leukemia (AML) and B-lineage acute lymphoblastic leukemia (B-lineage ALL) was compared with that on the myeloid and B-lymphoid CD34+ cells in normal bone marrow. Bone marrow aspirates of 10 patients with AML, 8 patients with B-lineage ALL and of 6 healthy volunteers were examined. The phenotype of the CD34+ cells was determined with a double immunofluorescence method and flow cytometry. CD34+ cells in AML and B-lineage ALL showed a lower expression of VLA-2 and VLA-3 and a higher expression of ICAM-1 and LFA-3 than their normal bone marrow counterparts. AML CD34+ cells had less L-selectin but more VLA-5 on their surface membrane than normal myeloid CD34+ cells. B-lineage ALL CD34+ cells showed an overexpression of LFA-3. In individual patients deficiencies or over-expression of the beta1 integrin chain, VLA-4, PECAM-1 or HCAM also occurred. An abnormal adhesive capacity of the leukemic cells may influence their proliferation, their localisation and apoptosis. An aberrant expression of adhesion molecules may be used for the detection of minimal residual leukemia in these patients.

Bone marrow stromal cells as a vehicle for gene transfer

Adoptive transfer of genetically modified somatic cells is playing an increasingly important role in the management of a wide spectrum of human diseases. Hematopoietic stem cells and lymphocytes have been used to transfer a variety of genes, however, they have limitations. In this study, the feasibility of retroviral gene transduction of bone marrow stromal cells, and the engraftment characteristics of these cells following infusion, was investigated in a murine transplantation model. Stromal cells derived from Balb/c mouse bone marrow were transduced with a replication-defective retrovirus containing the LacZ gene. Following three rounds of transduction, between 5 and 40% of the cells were positive for the LacZ gene. A total of 2 x 106 cells were infused into the same mouse strain. After the infusion, the LacZ gene was detected by PCR in the bone marrow, spleen, liver, kidney and lung; however, only the spleen and bone marrow samples were strongly positive. Quantitative PCR demonstrated that between 3 and 5% of spleen and bone marrow cells, and 1% of liver cells contained the LacZ gene at 3 weeks after infusion; <0.2% transduced cells were found in other organs. No difference was noted in engraftment between mice with or without irradiation before transplantation, suggesting that engraftment occurred without myeloablation. The infused transduced cells persisted for up to 24 weeks. Self-renewal of transplanted stromal cells was demonstrated in secondary transplant studies. Ease of culture and gene transduction and tissue specificity to hematopoietic organs (bone marrow, spleen, liver) is demonstrated, indicating that stromal cells may be an ideal vehicle for gene transfer.

Granulocyte colony-stimulating factor mobilized peripheral blood stem cells enter into G1 of the cell cycle and express higher levels of amphotropic retrovirus receptor mRNA

We compared the cell cycle status and expression of mRNA for the amphotropic retroviral receptor in hematopoietic stem cells isolated from bone marrow and cytokine mobilized peripheral blood. CD34+ cells from six normal volunteers were enriched by immune selection from steady-state bone marrow and granulocyte colony-stimulating factor (G-CSF) mobilized peripheral blood (10 microg/kg/day for 5 days). Cell cycle status of the phenotypically primitive CD34+CD38- hematopoietic stem cell population was analyzed using a four-color flow cytometry technique that distinguished the G0, G1, and S/IG2/M phases of the cell cycle. Semiquantitative reverse transcriptase-polymerase chain reaction was performed to measure mRNA expression of the amphotropic retroviral receptor. Peripheral blood hematopoietic stem cells had 2.6-fold more cells in the G1 phase of the cell cycle compared to steady-state bone marrow. Furthermore, lineage CD34+CD38- cells from G-CSF mobilized peripheral blood had a fourfold higher level of amphotropic retrovirus receptor mRNA. In conclusion, we found that CD34+ CD38- hematopoietic stem cells isolated from G-CSF mobilized peripheral blood differ from those isolated from steady-state bone marrow in that a significant proportion have entered the G1 phase of the cell cycle and express higher levels of amphotropic receptor mRNA. These biologic properties are consistent with the reported rapid recovery of hematopoietic function following transplantation with peripheral blood hematopoietic stem cells and make these cells a preferred target for retroviral-based gene transfer.

Biology of IL-8-induced stem cell mobilization

The CXC chemokine interleukin-8 (IL-8) has profound hematopoietic activities following systemic administration. It induces the rapid mobilization of cells with lymphomyeloid repopulating ability in mice and of hematopoietic progenitor cells in monkeys. In this paper, evidence is presented that stem cell mobilization in mice requires the functional expression on the beta 2-integrin leukocyte function-associated antigen-1 (LFA-1). In monkeys, systemic injection of IL-8 is followed by a significant increase in the circulating levels of the matrix metallo proteinase gelatinase-B (MMP-9). Based on these findings, the hypothesis is discussed that mature neutrophils serve as intermediate cells in IL-8-induced stem cell mobilization by the release of proteinases.

Cytoskeleton and integrin-mediated adhesion signaling in human CD34+ hemopoietic progenitor cells

Significant progress has been made recently in the understanding of cell adhesion signaling. Many components of focal adhesion complexes have been identified in fibroblasts and endothelial cells, showing considerable overlap and complementarity between growth signaling mediated by growth factor receptors and adhesive signaling mediated by cell adhesion receptors such as integrins. These studies showed that the cytoskeleton is essential for the correct intracellular localization of large signaling complexes that regulate the cellular machinery. Although adhesive interactions are essential to maintain steady-state hemopoiesis, the study of the function and role of adhesive interactions in hemopoietic progenitor and stem cells is less advanced. As in fibroblasts, functional overlap between hemopoietic growth factor receptors and cell adhesion receptors has been demonstrated, with the cytoskeleton likely playing a critical role in integrating information provided by soluble factors and cell adhesion molecules constituting the hemopoietic microenvironment. The intention of this article is to give a critical review of the current knowledge about the cytoskeleton and integrin-mediated signaling in hemopoietic progenitor cells.

Murine Hematopoietic Progenitor Cells With Colony-Forming or Radioprotective Capacity Lack Expression of the β2-Integrin LFA-1

Recently, we have demonstrated that antibodies that block the function of the β2-integrin leukocyte function-associated antigen-1 (LFA-1) completely abrogate the rapid mobilization of hematopoietic progenitor cells (HPC) with colony-forming and radioprotective capacity induced by interleukin-8 (IL-8) in mice. These findings suggested a direct inhibitory effect of these antibodies on LFA-1–mediated transmigration of stem cells through the bone marrow endothelium. Therefore, we studied the expression and functional role of LFA-1 on murine HPC in vitro and in vivo. In steady state bone marrow ± 50% of the mononuclear cells (MNC) were LFA-1neg. Cultures of sorted cells, supplemented with granulocyte colony-stimulating factor (G-CSF)/granulocyte-macrophage colony-stimulating factor (GM-CSF)/IL-1/IL-3/IL-6/stem cell factor (SCF) and erythropoietin (EPO) indicated that the LFA-1neg fraction contained the majority of the colony-forming cells (CFCs) (LFA-1neg 183 ± 62/7,500 cells v LFA-1pos 29 ± 17/7,500 cells,P &lt; .001). We found that the radioprotective capacity resided almost exclusively in the LFA-1neg cell fraction, the radioprotection rate after transplantation of 103, 3 × 103, 104, and 3 × 104 cells being 63%, 90%, 100%, and 100% respectively. Hardly any radioprotection was obtained from LFA-1pos cells. Similarly, in cytokine (IL-8 and G-CSF)–mobilized blood, the LFA-1neg fraction, which comprised 5% to 10% of the MNC, contained the majority of the colony-forming cells, as well as almost all cells with radioprotective capacity. Subsequently, primitive bone marrow-derived HPC, represented by Wheat-germ-agglutinin (WGA)+/Lineage (Lin)−/Rhodamine (Rho)− sorted cells, were examined. More than 95% of the Rho− cells were LFA-1neg. Cultures of sorted cells showed that the LFA-1neg fraction contained all CFU. Transplantation of 150 Rho− LFA-1neg or up to 600 Rho−LFA-1pos cells protected 100% and 0% of lethally irradiated recipient mice, respectively. These results show that primitive murine HPC in steady-state bone marrow and of cytokine-mobilized blood do not express LFA-1.

Murine Hematopoietic Progenitor Cells With Colony-Forming or Radioprotective Capacity Lack Expression of the β2-Integrin LFA-1

Recently, we have demonstrated that antibodies that block the function of the β2-integrin leukocyte function-associated antigen-1 (LFA-1) completely abrogate the rapid mobilization of hematopoietic progenitor cells (HPC) with colony-forming and radioprotective capacity induced by interleukin-8 (IL-8) in mice. These findings suggested a direct inhibitory effect of these antibodies on LFA-1–mediated transmigration of stem cells through the bone marrow endothelium. Therefore, we studied the expression and functional role of LFA-1 on murine HPC in vitro and in vivo. In steady state bone marrow ± 50% of the mononuclear cells (MNC) were LFA-1neg. Cultures of sorted cells, supplemented with granulocyte colony-stimulating factor (G-CSF)/granulocyte-macrophage colony-stimulating factor (GM-CSF)/IL-1/IL-3/IL-6/stem cell factor (SCF) and erythropoietin (EPO) indicated that the LFA-1neg fraction contained the majority of the colony-forming cells (CFCs) (LFA-1neg 183 ± 62/7,500 cells v LFA-1pos 29 ± 17/7,500 cells,P < .001). We found that the radioprotective capacity resided almost exclusively in the LFA-1neg cell fraction, the radioprotection rate after transplantation of 103, 3 × 103, 104, and 3 × 104 cells being 63%, 90%, 100%, and 100% respectively. Hardly any radioprotection was obtained from LFA-1pos cells. Similarly, in cytokine (IL-8 and G-CSF)–mobilized blood, the LFA-1neg fraction, which comprised 5% to 10% of the MNC, contained the majority of the colony-forming cells, as well as almost all cells with radioprotective capacity. Subsequently, primitive bone marrow-derived HPC, represented by Wheat-germ-agglutinin (WGA)+/Lineage (Lin)−/Rhodamine (Rho)− sorted cells, were examined. More than 95% of the Rho− cells were LFA-1neg. Cultures of sorted cells showed that the LFA-1neg fraction contained all CFU. Transplantation of 150 Rho− LFA-1neg or up to 600 Rho−LFA-1pos cells protected 100% and 0% of lethally irradiated recipient mice, respectively. These results show that primitive murine HPC in steady-state bone marrow and of cytokine-mobilized blood do not express LFA-1.